AN/ASN−92 INS Part I: Components

AN/ASN-92 INS Study: Table of Contents

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The INS has been mentioned several times on this website, especially the effects of Magnetic Variation and the Positional Fixes.
However, there is much more going on under the hood, and the relation between the navigation components of the avionics and carrier operations is more complex than it may look.
This series originates from a discussion with LynxOfTheSky (LynxOfTheSky#2969): he was looking deeper into the INS, and later sent me his findings. I found the observations about the AHRS quite interesting, sparking the desire to go deeper into the topic.
This first part introduces the main components of the Inertial Navigation System, what they do, and how they help the crew to complete their mission. Moreover, it attempts to provide more information useful in specific situations, such as the mentioned Carrier Operations, failures, and in general, which display or device is reliable in case the INS becomes unavailable.
Some of the conclusions may sound indeed surprising, and may affect your daily operations in the DCS’ skies.

INS: Meet the Team

The primary navigation system is the inertial navigation system (AN/ASN−92) that consists of the following components:

• Inertial Measurement Unit (IMU);
• Power Supply Unit (PSU);
• Pilot and RIO navigation controls and displays.

Additionally, the inertial navigation system operates with the AWG-9 Weapon Control System (WCS) computer and the Computer Signal Data Converter (CSDC).
Other associated equipment includes:

• Attitude and Heading Reference System (AHRS);
• Central Air Data Computer (CADC);
• Radar Altimeter;
• Instrument Landing System (ILS);
• TACAN.

AN/ASN−92 Inertial Navigation System

The INS is a dead−reckoning system, capable of deriving speed as a function of aircraft accelerations.
For this purpose, two accelerometers are used to quantify the acceleration in the horizontal place. The results are the two velocity components (X and Y) weighted for the Earth’s rotational velocity (see panel below) and integration inputs.
In the Inertial Measurement Unit, the velocities X and Y can be resolved through a wander angle, allowing the velocities to be expressed in the Earth-references North/East/Down system. Increments of latitude and longitude are provided by further integration of the North and East axes.
This process allows to provide the precise knowledge of the position of the aircraft at all times, along its direction and velocity.

Computer Signal Data Converter (CSDC)

The CSDC is composed by two analogue/digital converters (one analogue to digital and one digital to analogue) and a miniature general-purpose computer.
The CSDC is tasked to perform the inertial navigation computations after the INS alignment is completed. It is also the interface between the various navigation subsystems and the auxiliary equipment.

The Wander Angle

The CSDC calculates a value of wander angle from matrices representing two coordinate systems, the IMU, and the Earth’s coordinate system. This matrix represents the cosine of each system. The CSDC computes the angular relationship of the two systems during alignment, producing a very accurate value of wander angle, the difference between fighter reference line heading and true north.

Attitude and Heading Reference Set (AHRS)

The AHRS provides backup pitch and roll information to the CSDC and to the WCS computer in case the attitude information from the inertial navigation system are not provided.
The AHRS constantly supplies the prime magnetic heading value to the CSDC and to the BDHI for its analogue display, and heading reference to the autopilot.
The Bearing Distance Heading Indicator is the only analogue cockpit display showing the magnetic heading. The other displays located in the cockpit are digital, and receive their input from the AHRS through the CSDC (ECMD, TID, HSD, VDI and HUD). Consequently, in case of Computer Signal Data Converter failure, the only alternative display of (magnetic) heading is the BDHI.

AHRS Basic Components

The basic components of the Attitude and Heading Reference Set include:

• a two-gyro platform, composed by a vertical and a directional displacement gyro;
• an electronic control amplified;
• a compass controller.

Moreover, associated with the AHRS, there are the Magnetic Azimuth Detector (MAD) and an electronic compensator.
The platform consists of gyros, level sensors, gimbals, and related electronics. It is capable of unlimited roll, but it has a hard limit of 82° in pitch.
In case of IMU failure, the CSDC selects the attitude information from the AHRS automatically, and use them for both display and autopilot control.
The directional gyro usage depends on the selected mode (see Paragraph 6.11.3 for more information).

• In SLAVED mode, it is used to smooth the flux valve heading signal;
• in DG mode, it provides a direct heading reference.

The BDHI, the CSDC and the WCS use the resulting heading transmitted by the AHRS.

Additional Notes

• When the INS is in navigation mode, the true heading provided by the Inertial Measurement Unit is used. A backup value of the magnetic variation is calculated by adding or subtracting the magnetic variation to the true heading value provided by the IMU.
  In normal operations instead, the magnetic heading used for all magnetic displays is provided by the AHRS.
• The AHRS lacks an all-attitude capability, and it will precess (see box below) if the pitch attitude exceeds ±82°.
  In sustained turns at slow rates (less than 6° per minute), a gradual precession in roll, pitch and heading can also be expected.
  Large roll and pitch precession errors can be corrected by flying straight and level at a constant speed, and pressing and holding the HDG pushbutton on the Compass Control Panel (front seat) for at least 3 minutes. Allow a cooldown period of one minute before repeating the operation. Note that the F-14A and F-14B available in DCS do not feature a Digital Flight Control System. It was part of the F-14B(U) and the F-14D versions. The latter flew for the first time in 1995, and the application routine was scheduled to be completed in 2001
• In the event of IMU failure, the Digital Flight Control System (DFCS) uses the AHRS as a backup for the INS data to provide autopilot capability.
• If an undetected AHRS failure occurs, the Mv acronym will appear on the TID, signalling that an erroneous value of MAG VAR will be computed.
  Error events and troubleshooting will be discussed in another part of this series.

Magnetic Azimuth Detector

The magnetic heading value used by the aircraft is provided by the Magnetic Azimuth Detector. The MAD is interfaced with two components, a gyro and the electronic control amplified, which jointly work to stabilize and amplify the magnetic heading signals.
The MAD is in the left vertical tail section of the F-14 Tomcat, and it is commonly referred to as the flux value.

The Magnetic Azimuth Detector has a few important peculiarities, as highlighted by Super Grover:

[..] “the magnetic flux valve (Magnetic Azimuth Detector – MAD) in the vertical tail. It’s fixed. It feeds the AHRS with the relative direction of the magnetic North. When you roll or pitch, it becomes sensitive to the vertical component of Earth’s magnetic field. This means – it becomes erroneous. That’s why the directional gyro slaves to the MAD only in horizontal flight. At least in theory. In practice – you fly constant speed in a nose up attitude or with some hardly noticed bank for long enough, your directional gyro will slave to a slightly erroneous magnetic North.”

[..] “If your directional gyro/synchro is shifted, it may result in erroneous interpretation of the TACAN direction on all instruments. You might have the carrier straight in front of your nose, but the BDHI will show the TACAN a few degrees to your left or right. However, the radial (bearing) reading would be correct; it would be just your magnetic heading wrong.”

[..] “The magnetic flux valve alone is even simpler than a normal compass we know from the GA aircraft. In opposition to a normal compass, it is not balanced so will show the magnetic north only in horizontal flight. On the other hand, it is unaffected by acceleration errors because it has no moving parts.
It doesn’t have to be balanced, because, in normal operations, it is gyro stabilized.”

“When in a level flight at high AoA (let’s say 15 units), flying east or west, the magnetic flux valve behaves like a whiskey compass would behave when decelerating: it turns South. However, when flying north or south, it would turn much less or might even not turn at all. It is because in a nose up attitude, the flux valve is tilted back against the horizon, just like a whiskey compass when decelerating.

As I wrote above, the magnetic flux valve output isn’t presented directly to the crew – in normal operations, it is gyro stabilized in the AHRS. When flying low AoA, it should show more or less the correct magnetic heading. When you decelerate and fly high AoA, it will slowly align with the erroneous magnetic heading from the magnetic flux valve.

In our F-14, we simulate the magnetic field declination and inclination, the magnetic flux valve, the AHRS gyro stabilization and the AHRS slaving rates.”

Inertial Measurement Unit (IMU)

The core of the Inertial Navigation System is the IMN. It consists of a three-axis, four-gimbal, all-attitude unit containing accelerometers, gyro and the associated electronics.
Each part of the IMU has its purpose:

• the basic internal measurements necessary for the primary navigation computations are provided by the accelerometers;
• ensuring that the accelerometers are maintained in their proper orientation through all aircraft manoeuvres is the job of the four-gimbal structure.

To ensure proper functionality, the x-axis and y-axis accelerometers sense the local gravity and torque of the gyros, until the accelerometers outputs are zero. This ensures the IMU is leveled to the local vertical.
The WCS computer then uses the x- and y-accelerometer outputs whilst the CSDC processes the gyro-torquing computations, to calculate the wander angle and fine levelling.

Weapon Control System (WCS) Computer

Depending on the current navigation mode, the WCS uses input navigation data and selected store to perform the required navigational computations.
The WCS also performs an additional set of navigation computations:

1. Own−aircraft ground speed and ground track;
2. Range, bearing, command course, command heading, and time−to−go to selected destination positions;
3. Heading (true and magnetic);
4. Wind speed and direction (both true and magnetic);
5. Backup present position;
6. Magnetic variation (MagVar).

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Next part will introduce the Navigational Modes. Stay tuned!